We don’t think much about indoor air quality. Beyond the overall smell of a room – ah, fresh flowers or eww, dog breath – most of us just don’t consider it. But the quality of the air indoors impacts how we sleep, how we think, and most importantly, how we breathe. This is particularly true when we workout, when our respiration is exaggerated by vigorous exercise.

We all know that exercise and air pollution don’t mix. If you run or bike along a busy street, you risk increased exposure to dangerous fumes. This risk is exacerbated when we breathe open-mouthed, as we often do during aerobic activity, because we bypass the nasal passage which filters air-borne toxins. Headaches, eye and throat irritation, damaged airways, and worse can result. These deleterious effects are mitigated by timing your workout to correspond with less trafficked times, donning a mask, or working out indoors…Unless the indoor air quality is compromised and sometimes it is!

That smell at the gym is not always benign. Carbon dioxide levels rise in a packed gym. A greenhouse gas, carbon dioxide is a waste product of exercise. You exhale it. Though it is always present in your blood, too much of it can impair your nervous system and damage your respiratory system. Mold is another gremlin of indoor gym air. It thrives in humidity and can be associated with respiratory symptoms and immune deficiencies.

VOCs or volatile organic compounds, such as formaldehyde, fire retardants, acetone, also lurk indoors – especially in newer gyms where new paint or new carpeting have been installed. VOCs are a fairly common problem in modern buildings. And dirt. Regular old dirt, mostly in the form of dust, can cause respiratory problems and be prevalent in gyms and schools.

So, what is a fitness buff to do? Here are six things you can do right now to improve air quality during your workout:

Exercise in the best air you can find (you’ll know it by its fragrance)